The immunotherapeutic approach to the treatment of cancer is based on the observation that human tumor cells express a variety of tumor-associated antigens (TAAs) that are not expressed in normal tissues. These antigens, which include viral tumor antigens, cellular oncogene proteins, and tumor-associated differentiation antigens, can serve as targets for the host immune system and elicit responses which result in tumor destruction. This immune response is mediated primarily by lymphocytes; T cells in general and class I MHC-restricted cytotoxic T lymphocytes in particular play a central role in tumor rejection. Hellstrom, K. E., et al., (1969) Adv. Cancer Res. 12:167-223; Greenberg, P. D. (1991) in Advances in Immunology, vol. 49 (Dixon, D. J., ed.), pp 281-355, Academic Press, Inc., Orlando, Fla. Unfortunately, as evidenced by the high incidence of cancer in the population, the immune response to neoplastic cells often fails to eliminate tumors. The goal of active cancer immunotherapy is the augmentation of anti-tumor responses, particularly T cell responses, in order to effect complete tumor destruction.
Most attempts at active immunization against cancer antigens have involved whole tumor cells or tumor cell fragments. However, the cloning of TAAs recognized by CD8+ T cells has opened new possibilities for the immunotherapy of cancer based on the use of recombinant or synthetic anti-cancer vaccines. Boon, T., et al.,(1994) Annu. Rev. Immunol. 12:337-365; Brithcard, V., et al., (1993) J. Exp. med. 178:489-495; Cox, A. L., et al., (1994) Science 264:716-719; Houghton, A. N. (1994) J. Exp. Med. 180:1-4; Pardoll, D. M. (1994) Nature 369:357-358; Kawakami, Y., et al., (1994) Proc. Natl. Acad. Sci. U.S.A. 91:3515-3519; Kawakami, Y., et al., (1994) Proc. Natl. Acad. Sci. U.S.A. 91:6458-6462.
Two such antigens have been designated MART-1 (Melanoma Antigen Recognized by T cells-1) and gp100. Proc. Natl. Acad. Sci. U.S.A. 91:3515-3519. MART-1 and gp 100 appear to be expressed in virtually all fresh and cultured melanomas. With the exception of melanocyte and retina, no normal tissues express the antigens. The antigens may be responsible for mediating tumor regression in patients with advanced melanoma, since the tumor-infiltrating lymphocytes (TIL) used to identify MART-1 and gp100 were capable of effecting tumor regression in vivo. Thus, immunization of melanoma patients with MART-1 or gp100 may boost their cellular immune responses against their cancers.
The use of recombinant vaccinia viruses for anti-tumor immunotherapy has been reviewed. (Hu, S. L., Hellstrom, I., and Hellstrom K. E. (1992) in Vaccines: New Approaches to Immunological Problems (R. W. Ellis, ed) pp 327-343, Butterworth-Heinemann, Boston.) Anti-tumor responses have been elicited using a recombinant vaccinia virus expressing a TAA designated carcinoembryonic antigen (CEA). CEA is a glycoprotein expressed at high level on the surface of nearly all tumors of the gastrointestinal tract, as well as on many mammary carcinomas and lung adenocarcinomas. (Muraro, R., et al., (1985) Cancer Res. 4S:5769-5780.) A recombinant vaccinia virus that expresses CEA (Kantor, J., et al. (1992) J. Natl. Cancer Inst. 84:1084-1091) was evaluated using a murine tumor model in which the human CEA gene was transduced into murine colon carcinoma cells. (Robbins, P. F., et al. (1991) Cancer Res. 51:3657-3662.) Mice immunized with the CEA/vaccinia recombinant were resistant to the growth of subsequently transplanted CEA-expressing tumors. Moreover, when mice bearing established CEA-transduced murine carcinomas were treated with the recombinant virus, the tumors showed greatly reduced growth or complete regression. In rhesus monkeys, which carry an antigen on the surface of their granulocytes that cross-reacts with human CEA, immunization with the recombinant elicited anti-CEA antibodies, delayed type hypersensitivity, and lymphoproliferative responses. (Kantor, J., et al. (1992) Cancer Res. 52:6917-6925.) No toxicity was observed.
Prostate-specific antigen (PSA) is a 33,000-34,000 dalton glycoprotein that is produced in normal, benign, and cancerous prostate epithelia, but not in other normal or malignant tissues. (Wang, M. C., et al. (1982) Meth Cancer Res. 19):179-197.) PSA is secreted into prostatic fluid and seminal plasma. (See, Wang, et al.) Elevation of PSA levels in serum is correlated with growth of the prostate, and prostate cancer patients show an exponential increase in PSA levels. (Carter, H. B., et al. (1992) Cancer Research 52:3323-3328.) Due to its tissue specificity, PSA is a potential target antigen for immunotherapy against prostate cancer.
A number of laboratories have explored the use of recombinant poxviruses that express specific TAAs as immunotherapeutic vaccines. The ability of recombinant poxviruses expressing a variety of antigens to serve as potential vaccines for the prevention of infectious disease has been well-documented. Immunization with live recombinant pox virus allows expression of foreign antigens that are presented to the immune system together with highly immunogenic, virus proteins, which may act as adjuvants to enhance immune responses to the foreign antigen. Austin, F. C., et al. (1979) Adv. Cancer Res. 50:301-345. Finally, poxviruses are not oncogenic and do not integrate into the host cell genome, as replication and transcription of genetic material occurs in the cytoplasm of the infected cell.
Viruses of the family Poxviridae (pox viruses) are useful as vectors for the delivery of foreign genes and gene products in many clinical and research settings. Pox viruses of the genus Orthopoxvirus, particularly vaccinia, are used for several reasons. Among these are: (a) its wide use in humans in the eradication of smallpox; (b) its ability to infect a wide range of cells, including professional antigen presenting cells, and express the inserted gene product (i.e. foreign gene product) in a manner that has the potential to be processed in the context of class I and/or class II MHC molecules; and (c) use as a recombinant vaccine in the treatment of certain tumors (Kantor, J. et al. (1992)).
Fowlpox virus (FPV) is a member of the avipox virus family. Productive FPV infection is restricted in vivo to cells derived from avian species; however, FPV-mediated gene expression does occur in infected non-avian cells. Taylor, J. et al., (1988) Vaccine 6:497-503. Fowlpox virus based recombinant vaccines are described in: Technological Advances in Vaccine Development (Alan R. Liss, Inc.) pp. 321-334. Furthermore, in vivo FPV-mediated gene expression in several mammalian species has been demonstrated. Six non-avian species immunized with live recombinant fowlpox virus expressing the rabies glycoprotein developed antibodies against this glycoprotein. Immunization with this recombinant FPV elicited antibodies against this glycoprotein. Immunization with this recombinant FPV partially protected mice, cats, and dogs against a rabies virus challenge. There was no manifestation of proliferative infection or overt disease in any animals immunized with a variety of doses of live recombinant FPV.
Taylor, J., et al., (1988) Vaccine 6:497-503. In another study, a recombinant FPV containing a measles fusion protein was shown to partially protect mice against lethal challenge the measles virus, although antibodies against the fusion protein were not detected. Wild, T. F., et al., (1990) Vaccine 8:441-442. It was therefore postulated that protection was mediated by cellular immune responses. These results suggest that recombinant FPV may have utility as a safe and effective alternative to vaccinia virus as a vaccine vector.